Phospholipids are nonrandomly distributed across biological membranes. The choline-containing phospholipids are enriched on the plasma membrane outer leaflet, while the aminophospholipids are enriched on the cytofacial surface of the membrane. This asymmetry is maintained in part by a vanadate- and calcium-sensitive, MgATP-dependent, aminophospholipid transporter (a "flippase"). The goals of our work are to understand the mechanisms by which phospholipid asymmetry is maintained in human erythrocyte membranes and to determine the role that a loss of asymmetry plays in human cardiovascular disease. To this end we are pursuing three projects: The protein(s) responsible for aminophospholipid transport is being purified from human erythrocyte membranes. A MgATPase has been isolated that shares many characteristics in common with the aminophospholipid flippase. The ATPase is inhibited by calcium, vanadate and sulfhydryl reagents. It is selectively activated by phosphatidylserine (PS); other lipids support little or no ATPase activity. PS is recognized with high degree of structural and stereospecificity. Almost any modification to the functional groups on PS abolish the ability to activates the ATPase. Though L- and D-serine analogs activate ATPase activity equally well, only the naturally occuring sn-1,2-glycerol isomer supports ATPase activity. Recently, the ATPase has been reconstituted into proteoliposomes for studies of lipid transport. In future work, antibodies will be raised against the protein, it will be cloned, and biophysical studies of flippase structure and function will be performed. Numerous synthetic phospholipids and headgroup analogs of PS are required for this work. Each of these lipids has been characterized by FAB-MS, MS/MS and exact mass measurements.